At the time when blue collar was coined, most blue-collar workers weren’t required to wear any particular uniform or shirt color to work. While office workers could wear white-collared shirts without much fear of soiling them, and could also afford to launder their shirts regularly, manual laborers preferred darker colors. The German immigrant and frontier salesman Levi Strauss began to make denim in the 1870s, and the fabric quickly became popular with coal miners and other rugged Westerners. (Blue jeans wouldn’t become a middle-class institution until The Wild One, Rebel without a Cause, and the student protesters of the 1960s.) Chambray shirts, coveralls, boiler suits, and clothes made of dungaree also tend to come in blue, and these have been popular with manual laborers since the early 20th century. Office workers, for their part, moved away from wearing white in the 1960s. By 1970 about 80 percent of the shirts sold by Arrow, the country’s largest shirt manufacturer, were colored.

Explainer thanks Mary ThomasineHarkins of Emerson College and Barry Popik.

Correction, May 4, 2012:The original version incorrectly stated that collar colors weren’t used to discriminate by occupation until the 1930s. The phrase white-collar was used as early as 1910, and blue-collar had been coined by 1924at the latest.